Interview with Professor Andreas Bett, Director of the Fraunhofer Institute for Solar Energy Systems ISE
Solar energy has left fossil fuels behind in terms of economic efficiency. The market is buzzing with a steady stream of new solutions, with work on the next technological innovations already underway in the labs.
The solar industry is booming in Europe and companies like Meyer Burger and Next Wave are ramping up production in Germany and other European countries too. Why is Europe gaining in popularity again when it comes to production?
There are several reasons for this trend. Whilst photovoltaics are really cheap to produce, the transportation costs are on the rise. If you transport a module from China to Europe at the moment, that transportation will account for 10 % of the overall costs. It’s no wonder, then, that companies are keen to get production set up in Europe again.
The Fridays for Future movement has also caused a huge shift in society. With everyone talking more about sustainability and the energy transition, the number of new photovoltaic installations has seen a sharp increase. If you ask me, it’s still not enough, but it’s an encouraging sign that it’s once again worthwhile setting up production in Europe. I need to stress that we are looking at the entire value chain here. Wafer and cell production needs to be based in Europe and module production needs to be seriously ramped up.
Fraunhofer ISE is providing consultation for a project being run by Spanish startup Greenland, who are setting up a highly automated photovoltaic production line with an output of 5 gigawatts per year. Are similar projects on the horizon in Germany?
We certainly hope so. We already have production set up in Germany with Meyer Burger, which makes an important statement as far as future investors are concerned. Even 5 gigawatts is a mere fraction of the total capacity required by the European market, though. To give you an idea, we actually need to add 10 gigawatts a year in Germany alone to keep up with demand. We will still need to carry on importing too. That said, it’s essential that we hold onto our technological sovereignty in the energy industry.
If we were to just import modules, cells, and wafers for a decade, we would lose all our knowledge about the technological side of things. That’s certainly not the case right now. Our research institutes are still leading the way on a global level and we fully control mechanical engineering. But, going forward, it’s going to be crucial that we can take care of the industrial production here in Europe too. The coronavirus vaccination program has taught us just how important it is to have domestic production facilities that can be scaled up as required.
Over 90 % of solar cells on the global market are still made with crystalline silicon. Is that still state of the art?
In the industry, we have achieved an average annual increase in efficiency of 0.5–0.6 % at the cell level over the past ten years. That’s nothing short of impressive.
Back at the turn of the millennium, aluminum back surface field solar cells were the industry standard. The efficiency level ultimately peaked at 20 % before the industry found itself faced with structural limitations. There was simply no way of further improving the efficiency with that cell architecture, so new technology was the only answer. This essentially led us to new passivation coating, which complicated the solar cell structure and presented another challenge as far as process technology was concerned. While the necessary technologies had been developed in the labs, in the end, scaling up took the industry over 34 years. We need to keep that at the forefront of our minds now.
These structures, which we know now as passivated emitter and rear contacts or PERCs, allow us to achieve over 22 %. That’s the industry standard. And then we’re going to be limited by the structure again. We started researching passivating contacts in the labs around five years ago with a view to keeping losses to a minimum. At ISE, we coined the name TOPCon (tunnel oxide passivated contacts) for this. In fact, we have just recorded a new world record for a TOPCon structure with an efficiency level of 26 % achieved in the lab. That gives you an idea of the potential. And now this technology is gradually being rolled out across the industry.
The other major development route being explored in Europe is the heterojunction solar cell, which holds the promise of 26.5 % efficiency.
Which other innovations are there along the value chain?
What we are seeing a lot of right now is half-cut and third-cut cell modules. This development involves the wafer size being increased at the cell production stage, allowing for a higher throughput and in turn cutting costs. However, if the solar cells get too large, it becomes difficult to conduct electricity away. With that in mind, cells are separated again afterwards and connected up accordingly in the module. It’s obviously interesting to consider how best to connect them with as little active solar cell surface being lost as possible.
What do you think the future holds in the next five to ten years?
Theoretically, we can achieve a maximum efficiency level of 29.4 % with silicon as the semiconductor. In practical terms, we might be able to hit 27 %, but 26 % is the realistic maximum in an industrial setting.
Does that mean we’ve reached the end of the road with research and development?
No. We can stack a second semiconductor on top of the silicon solar cell to create tandem or multi-junction solar cells. To put it simply, we can use a solar cell material that is optimized to each color of the spectrum of sunlight. This strategy will allow us to push past the structural barriers of silicon. We recently recorded an efficiency rating of 35.9 % in the lab this way. We had a silicon solar cell at the bottom followed by two different III-V semiconductors stacked on top of one another. There was no difference from the outside, but the cell had an efficiency rating of 35.9 %. Now we obviously need to work on the industrial applications of this technology.
Could we replace silicon with another material that may work even better?
Perovskite crystals have been identified as an effective material for solar cells over the past five or six years. And the rapid research behind them has delivered significant increases in efficiency. If perovskite crystals or layers are used – even without a silicon base layer – an efficiency rating of 25 % can already be achieved. Our research labs are now looking into the options of pairing perovskite with silicon in tandem solar cells, using perovskite-only solar cells, and creating all-perovskite tandem solar cells.